Summary of work: 1) We have continued our work on the mechanisms underlying Hebbian, activity dependent synapse elimination in an in vitro neuromuscular synaptic system. When one input to doubly innervated myotubes is stimulated there is a selective loss of the other, inactive input to the same myotubes. We have previously shown, by the use of appropriate inhibitors, that this process is dependent on the action of protein kinase C (PKC). Cholinergic stimulation of muscle increases PKC. We now show that the kinase acts postsynaptically and has a major effect on the muscle acetyl choline receptor(AChR). Physiological analysis with intracellular recordings and utilizing a variance method of determining the locus for changes in synapse efficacy shows that the effect of PKC activation (by phorbol ester treatment) is a decrease in post-synaptic responsiveness, with no detectable change in the presynpatic transmitter release mechanism. The size of the unitary synaptic quantal event is also reduced by PKC activation. These changes are accompanied by a corresponding decrease in the concentration of AChR in the muscle surface membrane without detectable presynaptic anatomical change. An activity dependent synapse stabilization or augmentation is a crucial component of any Hebbian model of synaptic plasticity. We find that when synapse loss is induced by TPA this can be blocked (synapses preserved) by low frequency stimulation of the synapses. This preserving effect of activation is blocked by a PKA inhibitor, H89, as well as another PKA blocker, Rp-cAMPS. We now show that the preserving effect of activation can be demonstrated as an increase in the end plate potential (EPP) and a preservation of quantal amplitude, and can be mimicked by the application of cyclic adenosine monophosphate (cAMP). CAMP also prevents the loss of AChR from the muscle membrane produced by PKC activation This suggests that the positive aspect of the Hebbian model requires the action of PKA. Considerable evidence from the literature indicates that both PKC and PKA phosphorylate the AChR and the differential phosphorylation produced by these two kinases results in different effects on AChR stability. PKC destabilizes while PKA stabilizes the receptor. The specificity of loss of non-stimulated inputs and retention of stimulated inputs we hypothesize to be due to differential localization of the A and C kinases. PKA anchoring proteins occur at the neuromuscular junction and future work will focus on showing that the appropriate localization of the kinases exists and spatial patterns of their activation can generate the activity specific synaptic modulation that is characteristic of the widely invoked Hebbian synaptic plasticity.